(1) Field of the Invention
The invention relates to processes and to a device for determining the actual position of a structure of an object to be examined in a coordinate system.
(2) Description of Related Art including information disclosed under 37 CFR 1.97 and 1.98
Computed tomography scanners serve to create three-dimensional images, namely, so-called CT scans, of objects such as, for example, workpieces or human bodies or body parts, whereby these images also show internal structures of the object.
In computed tomography technology, abbreviated as CT technology, X-rays are used to take images of an object or of a part thereof from many different directions, that is to say, the object is X-rayed sequentially from many different directions. Thus, a computed tomography scanner, hereinafter referred to as a CT scanner, has an X-ray source and a two-dimensional position-resolving detector, for example, a CCD matrix, that is sensitive to the radiation emitted by the X-ray source. The X-ray source emits radiation, typically, for instance, of 450 keV. The object is positioned between the X-ray source and the detector and it is rotated incrementally with respect to the X-ray source or to the screen, or conversely, the CT scanner is rotated incrementally around the object, which in this case remains stationary.
When it is not the CT scanner but rather the object that is rotated, the CT scanner generally has an object support stage on which the object is placed. The object support stage can travel in such a way that the object can be positioned in the beam path of the X-rays. Moreover, the object support stage is rotatable so that the object can be rotated in order to create the CT image. Typically, the object is rotated, for example, in increments of 0.9° each, so that 400 rotation steps amount to a full revolution of the object. In this manner, the CT scanner can detect a certain volume that comprises either the entire object or a part thereof.
For each of the rotational positions that the object passes through, the detector takes a two-dimensional transmission X-ray image of the object. On these two-dimensional images, the object appears larger than it is in reality since its size on the image corresponds to a centered projection of the object on the detector surface, whereby the X-ray source is the center of projection.
Based on the array of two-dimensional individual images obtained in this manner, a computer is used to calculate a three-dimensional digital image of the volume detected by the CT scanner, i.e. of the object or of a part thereof, said image also showing internal structures that are completely enclosed in the object, insofar as these internal structures have an absorption coefficient for the radiated X-rays that differs from their surroundings, which is the case, for example, with cavities such as drilled holes.
After each full revolution of the object, the object support with the object can be shifted translatorily by a certain distance and the process explained above can be carried out again.
Such a commercially available CT scanner is described, for example, in the brochure: “RayScan 3D-X-Ray Computed Tomography”, PRO-RS-A-E000 11/01, made by Hans Walischmiller GmbH, D-88677 Markdorf, Germany.
Such CT scanners can be used to examine the internal structures of objects, for example, drilled holes in workpieces. In order to examine a structure of the object in this manner, first of all, a CT image of the entire object, including the structure of interest, can be created.
A drawback here is that, as a rule, the image of the structure only occupies a small part of the image field of the CT scanner since the CT scanner can only achieve a limited relative spatial resolution. This is especially due to the finite diameter of the exit pupil of the X-ray source and to the limited number of pixels of the CCD matrix; typically, a relative lateral resolution of, for example, 1:4000 can be achieved.
Therefore, in this case, a structure of the object whose extension is, for example, 1% of the object size, is only imaged at a relative resolution of 1:40 which, in most cases, is insufficient for a detailed examination of the structure.
Therefore, the CT image of the object can be used to determine the location of the structure within the object with respect to the coordinate system of the CT scanner and to carry out a second CT scan of the object, while regulating the CT scanner in such a way that only the immediate vicinity of the structure is detected by the CT scanner and a second CT image of this vicinity is made with a greater magnification factor. In this manner, the relative resolution of the image of the structure is enhanced, that is to say, more details of the structure become visible.
However, this also entails disadvantages. For example, it is a demanding procedure to determine the location within the object on the basis of the first CT image. Moreover, such a localization of the structure is imprecise since not only the structure itself but also the surface of the object can only be detected with the limited resolution of the CT scanner, as a result of which the appertaining measuring uncertainties become greater.
Moreover, additional time is required to create the second CT image. In view of the very high operating costs of a CT scanner, this need for additional time is a substantial cost factor in examining the structure.
Furthermore, in order to create the second CT image, the object is once again exposed to a certain dose of ionizing radiation. This is especially disadvantageous or problematic if the object consists of living biological matter. The repeated radiation exposure can also have a detrimental effect on non-living material. Ionizing radiation can trigger, for example, ageing, transformation, discoloration or degradation of plastics, it can have an altering effect on crystal structures or it can destroy electronic modules.